1. Field of the Invention
The present invention relates to a method for improving a MIMO procedure for a wireless communications system and related communications device, and more particularly to a method for improving a MIMO procedure about de-activation of a MIMO operation for a user equipment (UE) of a wireless communications system and related communications device.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. Through the 3G mobile telecommunications system, a user can utilize a wireless communications device, such as a mobile phone, to realize real-time video communications, conference calls, real-time games, online music broadcasts, and email sending/receiving. However, these functions rely on fast, instantaneous transmission. Thus, targeting at the third generation mobile telecommunication technology, the 3rd Generation Partnership Project (3GPP) provides High Speed Package Access (HSPA) technology, which includes High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA), to increase bandwidth utility rate and package data processing efficiency so as to improve uplink/downlink transmission rate.
To further increase the downlink data rate, 3GPP introduces a Multi-input Multi-output (MIMO) technology, with which a user equipment (UE) and a base station, known as a Node-B, utilize multiple antennas to transmit/receive radio signals. Besides, the MIMO technology can be further incorporated with spatial multiplexing, beam forming and spatial diversity technologies to reduce signal interference and increase channel capacity. In the prior art, the MIMO operation is mainly employed to the HSDPA system, doubling the peak rate of high speed downlink share channel (HS-DSCH). Concerning radio resource control (RRC) states of the UE, the MIMO operation is only applicable for the UE in CELL_DCH state.
To control MIMO operation of the UE, a universal terrestrial radio access network (UTRAN) can set configuration of MIMO operation in information elements (IEs) of RRC messages and send the RRC messages and IEs to the UE through corresponding RRC procedures. According to an RRC protocol specification of 3GPP, a MIMO parameter IE, having MIMO configuration data for the UE, can be included in reconfiguration messages, such as RRC CONNECTION SETUP, ACTIVE SET UPDATE, CELL UPDATE CONFIRM messages. On the other hand, the UE includes a MIMO_STATUS variable to store the MIMO configuration data included in the MIMO parameter IE. When the UE activates the MIMO operation, the UE needs to trigger lower layers to start operation in a MIMO mode according to the MIMO_STATUS variable.
During the MIMO operation, if radio link failure occurs, or a radio link control (RLC) unrecoverable error occurs, or transmission of a UE CAPABILITY INFORMATION message fails, the UE will initiate a cell update procedure to remedy the situations. When a cell update procedure is initiated, the UE shall enter CELL_FACH state to select a suitable cell. That is, the UE transits from CELL_DCH state to CELL_FACH state during the MIMO operation.
However, according to the prior art, the MIMO operation is only applied to a UE in CELL_DCH state. Furthermore, the prior art does not specify the UE action about leaving CELL_DCH state during MIMO operation. Thus, the above situations can occur system malfunction in the UE.